Group III-nitride coated substrates are increasingly valued for their usefulness in preparation of electrical components. For example, Group III-nitride based semiconductors have become a leading material for use in the production of light emitting diodes (LEDs), particularly blue laser diodes, as well as high power, high temperature electronics. While Group III-nitride coatings have proven extremely useful and versatile, they have also proven somewhat difficult to prepare. Growth of such coatings is generally through epitaxial methods, such as Metal Organic Vapor Phase Epitaxy (MOVPE) or Molecular Beam Epitaxy (MBE).
Epitaxial methods for depositing Group III-nitride films are not without their problems. For example, epitaxial growth can be hindered by the lack of a single crystalline Group III-nitride substrate, or other high quality single crystalline substrates, with the same lattice parameters as the Group III-nitride film to be grown. Epitaxial growth of nitrides is often performed on sapphire or silicon carbide substrates. Even with such substrates, though, problems can arise due to lattice mismatch between the nitride epi-layer and the substrate.
The properties of the Group III-nitride film are also known to vary depending upon the polar orientation of the film. Group III-nitride films crystallizing in a wurtzite crystal structure possess a polar axis oriented along the (0001) direction, as illustrated in FIG. 1. The polarity arises from the non-centro symmetric crystal structure of the Group III-nitrides. The most common growth direction of the Group III-nitrides is normal to the (0001) basal plane, where the atoms are arranged in bilayers consisting of two closely spaced hexagonal layers, one containing the cations and the other the anions, so that the bilayers have polar faces, as shown in FIG. 1. The polar structure with three bonds of the Group III atom facing toward the substrate and the single bond facing away from the substrate is commonly referred to as being Group III-polar. Accordingly, the mirrored structure with three bonds of the Group III atom facing away from the substrate and the single bond facing the substrate is commonly referred to as being Nitrogen-polar (or N-polar). When a multi-plane substrate, such as sapphire, is used, the polar orientation can also be described in terms of the substrate plane. For example, when using c-plane sapphire as the substrate, the Group III-polar orientation can be referred to as the +c orientation, while the N-polar orientation can be referred to as the −c orientation, as illustrated in FIG. 2.
Polar orientation is not to be confused with surface termination of the film components, as each orientation may be terminated with either one of the species comprising the film (e.g., gallium or nitrogen in a GaN film). Rather, polar orientation determines the direction of the spontaneous polarization vector and, thus, determines the type of charge induced at the surface/interface. Along with the piezoelectric polarization, the polarization-induced charge influences the electrical and optical properties of the coated substrate. Control of the polar orientation on a macroscopic and microscopic scale is desirable to exploit the combined properties of both types of orientations.
Given the usefulness of the Group III-nitride films, and components incorporating such films, and the different properties inherent to the polar orientations of the films, it is desirable to have a method for preparing a Group III-nitride film of controlled polarity. Further, it would be useful to have a method for preparing a dual polarity Group III-nitride film. Such methods, as well as Group III-nitride film coated substrates of specified polar orientation are provided according to the present invention.